Fiber coupler system and associated methods for reducing back reflections

ABSTRACT

An apparatus which couples light to a fiber from a light source at an input plane while reducing back reflections includes returning light reflected back through such that the returning light does not substantially overlap with an output of the light source in the input plane. This apparatus may include a mode matching element and/or an angular distribution altering element. The apparatus may be reciprocal. The reciprocal apparatus may prevent light traversing the apparatus again having a change in phase of light from substantially overlapping an original object in an input plane.

CROSS-REFERENCES TO RELATED APPLICATIONS

The present application is a continuation-in-part of U.S. applicationSer. No. 09/329,996 entitled “A Multi-Mode Fiber Coupler, System andAssociated Methods” filed Jun. 11, 1999, which claims priority under 35U.S.C. §119 to Provisional Application No. 60/101,367 filed on Sep. 22,1998, the entire contents of both of which are hereby incorporated byreference for all purposes.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention is directed to a coupler for coupling light to afiber while minimizing feedback to the light source due to reflectionalong the transmission system.

2. Description of Related Art

As the use of non-physical contact connections between light sources andfibers increases, the need for effective isolation to prevent lightreflected at the fiber interface from being returned to the light sourceincreases. Feedback to the light source may result in spectralbroadening, light source instability, and relative intensity noise,which affect the monochromaticity of the light source. As data rates goup, the systems become more sensitive to relative intensity noise andrequire low bit error rates. Conventional optical isolators usingpolarization effects to attenuate reflection are very expensive, makingthe non-physical contact impractical. The importance of avoidingfeedback is further increased when trying to use cheaper light sources,such as vertical cavity surfaces emitting laser diodes and lightemitting diodes.

One solution that avoids the use of an optical isolator is a modescrambler that divides power from the light source into many modes. Aconfiguration employing a mode scrambler includes a single mode pigtailthat provides light from the light source to the mode scrambler thatthen delivers the light to a transmission cable via an air-gapconnector. Since any reflected power will still be divided across themany modes, any reflected power in the mode that can efficiently becoupled into the pigtail is only a small fraction of the total reflectedpower, thereby reducing return losses. However, this solution involvesaligning another fiber, physically contacting the fiber with the modescrambler, and placing the light source against the fiber. Thispigtailing is expensive. Thus, there still exists a need for truenon-physical contact connection between a light source and atransmission system that does not require an isolator.

SUMMARY OF THE PRESENT INVENTION

The present invention is therefore directed to a coupler between a lightsource and a transmission system that substantially overcomes one ormore of the problems due to the limitations and disadvantages of therelated art.

These and other objects of the present invention may be realized byproviding an apparatus for coupling light from a light source at aninput plane into an optical fiber including an optical element whichcouples light from the light source to the optical fiber and whichreturns light traversing the optical system back towards the input planesuch that the returning light does not substantially overlap with anoutput of the light source in the input plane.

The optical element may be a mode matching element, wherein light outputfrom the optical element is distributed in a desired angulardistribution that is substantially maintained along the fiber for morethan a depth of focus of the optical element. The mode matching elementmay be a diffractive element. The mode matching element may be arefractive element. The optical element may have first and secondsurfaces, with the mode matching element being provided on the secondsurface, further from the light source. The apparatus may furtherinclude an angular distribution altering element on the first surface.The angular distribution altering element may provide a ring pattern onthe second surface. The angular distribution altering element may be adiffractive surface having a radially symmetric lens function and anegative axicon function.

The optical fiber may be a multi-mode GRIN fiber and the optical elementcouples light into higher order modes of the multi-mode GRIN fiber toreduce differential mode delay. The optical element may be a diffractivesurface having a radially symmetric lens function and a negative axiconfunction. The optical element may map a point from the input plane intomore than one point on the optical fiber. The optical element may couplelight to the optical fiber by directing light from the light source awayfrom a center of the optical fiber.

The above and other objects of the present invention may be realized byproviding a system for coupling light to an optical fiber including alight source at an input plane and an optical element which coupleslight from the light source to the optical fiber and which returns lighttraversing the optical system back towards the input plane such that thereturning light does not substantially overlap with an output of thelight source in the input plane.

The system may include a power monitor for the light source. The powermonitor may monitor light output from a front of the light source. Thesystem may include a deflecting element that deflects a portion of thelight output from the front of the light source to the power monitor.

The above and other objects may further be realized by providing anapparatus including a reciprocal optical element that couples light froma light source in an input plane into a fiber and reduces coupling oflight reflected back to the light source. The reciprocal optical elementmay be a single optical element. The single optical element may have atleast two surfaces. The reciprocal optical element may be an axicon. Thereciprocal optical element may be a mode matching element. Thereciprocal optical element may map a point in the input plane to morethan one point at the fiber. A change in phase of light passing backthrough the reciprocal optical element towards the light source mayprevent light from substantially overlapping an output of the lightsource in the input plane.

While the present invention is described herein with reference toillustrative embodiments for particular applications, it should beunderstood that the present invention is not limited thereto. Thosehaving ordinary skill in the art and access to the teachings providedherein will recognize additional modifications, applications, andembodiments within the scope thereof and additional fields in which theinvention would be of significant utility without undue experimentation.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other objects, aspects and advantages will bedescribed with reference to the drawings, in which:

FIG. 1 shows the integration of the coupler of the present inventionwith a light source, a fiber and a light source power monitor;

FIGS. 2A-2C illustrate a diffractive element and associatedcharacteristics of a spiral generator for use as the coupler inaccordance with one embodiment of the present invention;

FIG. 3 is a schematic illustration of another embodiment of the couplerof the present invention; and

FIG. 4 is a schematic illustration of another embodiment of the couplerof the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 1 illustrates a light source 10, here a VCSEL, a coupler 12 and amulti-mode fiber 14 integrated with a power monitor 16 and a reflectivesurface 18 for directing the light into the fiber 14. In particular, thelight source 10 and the power monitor 16 are provided on a substrate 20.Another substrate 22 has the coupler 12 thereon, preferably on the facefurthest from the light source to allow the beam to expand, and asplitting diffractive element 24 which splits off a portion of the lightfrom the light source 10 to be monitored. The substrates 20, 22 arepreferably mounted with spacer blocks 26, which provide the desiredseparation between the substrates 20, 22. The coupler 12 may also beprovided in a common housing with the fiber 14.

The light split off by the diffractive element 24 is directed to thepower monitor 16 to monitor the operation of the light source 10. Thedirected of the light to the power monitor 16 may be achieved byproviding appropriately positioned reflective portions 28. The number oftimes the light to be monitored traverses the substrate 22 is a designchoice, depending on the initial angle of diffraction and the desiredpositioning of the power monitor 16. This monitoring is set forth incommonly assigned, co-pending U.S. application Ser. No. 09/386,280,entitled “A Diffractive Vertical Cavity Surface Emitting Laser PowerMonitor and System” filed Aug. 31, 1999, which is hereby incorporated byreference in its entirety for all purposes. Alternatively, the powermonitoring may be realized using an integrated detector, without theneed for the deflecting element, as set forth in commonly assigned,co-pending U.S. application Ser. No. 09/548,018, entitled “TransmissionDetection for Vertical Cavity Surface Emitting Laser Power Monitor andSystem” filed Apr. 12, 2000, which is hereby incorporated by referencein its entirety for all purposes.

The light that is not split off by the diffractive element 24 proceedsto the coupler 12. A reflective surface 18, such as a polished angularface of another substrate, is provided to direct the light from thecoupler 12 into the multi-mode fiber 14. Preferably all the opticalelements are formed lithographically and all the elements are integratedon a wafer level.

In accordance with the present invention, the coupler 12 is adiffractive element that matches the phase as well as the intensitydistribution of the beam. The matching of the phases generates spiralpropagation of the beam through the fiber. This spiral or vortexpropagation maintains the intensity profile input to the fiber along thefiber. Since the beam travels in a corkscrew, the amount of lightcrossing the center of the fiber is significantly reduced. Ideally, theamount of light in the center will be zero, but in practice, the amountof light is on the order of 10% or less. In contrast, when only theintensity distribution is controlled, as in the first two designs of theparent application, the input intensity profile may be the desiredprofile, but will quickly degrade as the light traverses the fiber. Inother words, while the other designs may provide an input profile thatis substantially null on axis, this profile is only maintained for thedepth of focus of the coupler. When also matching the phase, thisprofile is maintained substantially beyond the depth of focus of a lenshaving the same numerical aperture as the beam to be input to the fiber,e.g., at least an order of magnitude longer. Absent the fiber, the nullspace of the beam profile is maintained through free space, whichsignificantly reduces the alignment requirement. Further, by matchingthe phase and amplitude of the beam to a certain mode of the fiber,theoretically the beam profile could be maintained over an infinitelength of fiber. However, imperfections in the real world, e.g., in thefiber, in the beam, in the matching, degrade from this theoreticalscenario.

Thus, in order to avoid low order modes in a GRIN fiber launch, theamplitude and phase of the higher order modes need to be matched. Thefollowing equations are set forth in Fields and Waves in CommunicationElectronics, Simon Ramo et al. 1984, particularly pp. 765-768, which ishereby incorporated by reference in its entirety. For a GRIN fiber,these eigenmodes all have the form set forth in Equation (1):

E(r,θ,z)∝ƒ_(mp)(r)e ^(±jmθ) e ^(±jβ) ^(_(mp)) ^(z)  (1)

where ƒ(r) is a function that depends only on r for given modes within aspecific fiber, r is the radius from the axis, θ is the angle from theaxis, z is the distance along the axis, m is the azimuthal mode number,β is a propagation constant, p is the radial mode number. When m, p=0,the beam has a Gaussian profile.

While Equation (1) could be used to match a particular mode of the fiberby creating an input light beam having an amplitude and phase functionwhich exactly correspond to the particular mode, such matching is notrequired and may not even be desirable, as matching the amplitude aswell as the phase increases the requirements on the optics. As long asm>0, the azimuthal mode m will have a phase function that is a spiralring, whether the light is traveling in free space or in a fiber. Oncethe phase function for at least one higher order mode, i.e., m>0, hasbeen matched, a null at the center of the beam is created after the beamhaving been phase matched propagates over a short distance, e.g., a fewwavelengths. Unlike other types of matching, this null is maintained inthe center in both free space and the fiber, so such an optical elementproviding such matching does not have to be immediately adjacent to thefiber. As evident from Equation (1), when matching the phase, the valueof p doesn't matter.

In order to suppress the lowest order mode, i.e., m=0, a phase termneeds to be added to the wavefront. This is accomplished through the useof the following diffractive phase function encoded onto the wavefrontset forth in Equation (2):

φ(x,y)=m arctan(y/x)  (2)

where φ is the phase function, x and y are the coordinates in the plane.In general, there will be several modes propagating, e.g., m=1-5. Thespiral mode may be realized by matching the phase function for m=3.

This phase function can be added to a lens function and encoded as amod(2π) diffractive element as set forth in Equation (3):$\begin{matrix}{{\varphi \left( {x,y} \right)} = {\frac{\pi \left( {x^{2} + y^{2}} \right)}{\lambda \quad f} + {m\quad \arctan \quad \left( \frac{y}{x} \right)}}} & (3)\end{matrix}$

FIG. 2A illustrates the mod(2π) diffractive element and thecorresponding intensity in the focal plane of the lens function. FIG. 2Billustrates an actual example of a diffractive optical element 12created in accordance with Equation (3). FIG. 2C illustrates thesimulated ring intensity 25 and the measured intensity pattern 29 of theelement 12 in FIG. 2B. A refractive equivalent in accordance withEquation (3) of the phase matching diffractive 12 may be alternatelyemployed.

This phase matching coupler 12 is not a true beam shaper, since eachpoint in the input plane is mapped into more than one point in theoutput plane because of the axial singularity. Unlike a diffuser, eachpoint in the input plane is not mapped to every point in the outputplane.

The phase matching coupler 12 allows the desired angular distribution tobe substantially maintained along a portion of the fiber. This may bequantified by measuring the amount of power within a certain radius ofthe fiber at a certain distance along the fiber. The phase matching ofthe present invention allows more power to be contained within thedesired radii for a longer distance than methods not employing phasematching. For example, by aligning the coupler and a GRIN fiber alongthe same axis, using a 850 nm source, and matching both the phase andthe amplitude, the encircled energy can be maintained to less than 12.5%is a radius of less than 4.5 microns and 75% for a radius less than 15microns, with no power in the fiber center, for over 6 m.

By matching the phases, the light from the coupler is input to the fibertraveling in a circular direction, i.e., the path of the light down thefiber forms a corkscrew. Such traversal is opposed to the linear travelnormally occurring down the fiber. By traveling in a corkscrew or spiralmode, the input distribution, typically annular, of the input light ismaintained along the fiber. Without the phase matching, while theinitial input light has the desired shape, this shape is not retainedthroughout the traversal of the fiber. Therefore, more modal dispersionwill be present, with more light in the center of the fiber, if phasematching is not used.

In addition to efficiently coupling the light into the fiber, the phasematching coupler 12 also reduces the power being fedback into the lightsource 10. Since the phases are matched, and the reflected light willnot have the same phase as it did when originally incident on the phasematching coupler 12, the phase matching coupler 12 will not return thelight back to the light source as it came. In other words, when thereflected light traverses the system, it will be further deflected bythe phase matching coupler 12, thereby reducing the power fedback intothe light source 10.

The back reflection reduction of the phase matching coupler onlyoperates sufficiently when the phase matching coupler 12 is far enoughaway from the fiber so that the phase is sufficiently changed to preventbeing redirected in the same manner. In other words, if the phasematching coupler 12 is placed in contact with the end of the fiber,while the coupler will still serve to maintain the input distribution,since the reflected light will have essentially the same phase as theinput light, the light will be returned substantially back to the lightsource as it came. However, if the phase matching coupler 12 is placedat least roughly three times the diameter of the beam incident on thefiber, there is sufficient alteration of the phase due to traversal thatthe reflect light incident on the phase matching coupler 12 will befurther deflected.

Further reductions to the amount of light being fedback to the lightsource 10 may be realized by using a lens 30 in addition to the phasematching coupler 12 as shown in FIG. 3. This lens 30 is used to shapethe light to provide additional reduction in the power fedback to thelight source. The lens 30 is preferably a diffractive surface that is acombination of a lens function having radially symmetric terms with anegative axicon function. When the phase matching coupler 12 is spacedaway from the fiber, the lens 30 may simply form a ring, since the phasematching coupler will prevent the light from being retraced. As shown inFIG. 3, the lens 30 is on a first surface 34 of a wafer 32. The phasematching coupler 12 is provided on a second surface 36 of the wafer 32,opposite the first surface. The thickness of the wafer 32 controls thenumerical aperture of the image. Alternatively, the phase matchingcoupler 12 may be formed on the same surface as the lens 30.

The lens 30 allows an annular intensity ring to be optimized for theparticular fiber 14. Also, by forming this ring prior to the phasematching coupler 12, a smaller radial segment of the phase matchingcoupler is used. As can be seen from equation (2), as m increases, theamount of phase twist increases. Thus, rays at the center of the phasematching coupler 12 receive a larger skew angle that rays at the edge ofthe phase matching coupler. By shaping the light into an annulus, thiscentral portion is avoided, reducing the aberrations introduced by thephase matching coupler 12. Again, the light reflected back from thefiber 14 will not have the same phase as the light incident on the phasematching coupler 12, so the light will be further deflected by the phasematching coupler 12. Since the deflection angles are now altered fromthat of the light source, the lens 30 will not focus the light back ontothe light source, but will further deflect the light away from the lightsource.

Another embodiment is shown in FIG. 4. Here, the phase matching coupler12 is not used, only a reciprocal, phase sensitive system 40. An opticalelement will map an optical distribution, i.e., amplitude and phasedistribution in an input plane to an output plane. If an optical elementis a reciprocal optical, it will map the same optical distribution in anoutput plane back to the original optical distribution in the inputplane, as long as the light has the same phase and intensity profile.Optical systems that perform one-to-one mapping, such as an imaginglens, are reciprocal, but are also phase insensitive when performing amapping between an object plane and an image plane, i.e., a change inphase will not affect the mapping between the image and object planes.However, other optical systems, such as those that perform a one to manymapping, i.e., in which one point in the input plane is mapped to morethan one point in the output plane, while reciprocal, are typicallyphase sensitive. In other words, a phase change will alter how the lightin the output plane is returned to the input plane. An example of such asystem is a negative axicon.

In the preferred embodiment, this system 40 also creates an intensityring on the plane at which the fiber 14 is located. The reflection fromthe fiber creates a ring back onto the system 40, but the phase of thelight has been altered due to the reflection. This change in phaseresults in the light traversing the system 40 having an increaseddiameter of the ring in the object plane, rather than returning the ringto the point source of the light source. This increased diameter resultsin most of the light missing the input of the light source,significantly reducing feedback. Any other reciprocal, phase sensitivesystem that results in most of the light avoiding the light source maybe used. The phase matching coupler 12 may still be employed in anyposition to increase coupling bandwidth and/or enhance the feedbacksuppression.

While the present invention is described herein with reference toillustrative embodiments for particular applications, it should beunderstood that the present invention is not limited thereto. Thosehaving ordinary skill in the art and access to the teachings providedherein will recognize additional modifications, applications, andembodiments within the scope thereof and additional fields in which theinvention would be of significant utility without undue experimentation.

What is claimed is:
 1. An apparatus for coupling light from a lightsource at an input plane into an optical fiber comprising an opticalelement which couples light from the light source to the optical fiberand which returns light traversing the optical system back towards theinput plane such that the returning light does not substantially overlapwith an output of the light source in the input plane, wherein saidoptical element is a mode matching element, wherein light output fromthe optical element is distributed in a desired angular distributionwhich is substantially maintained along the fiber for more than a depthof focus of the optical element.
 2. The apparatus of claim 1, whereinsaid mode matching element is a diffractive element.
 3. The apparatus ofclaim 1, wherein said mode matching element is a refractive element. 4.The apparatus of claim 1, wherein said optical element comprises firstand second surfaces, said mode matching element being provided on saidsecond surface, further from the light source.
 5. The apparatus of claim4, further comprising an angular distribution altering element on saidfirst surface.
 6. The apparatus of claim 5, wherein said angulardistribution altering element provides a ring pattern on said secondsurface.
 7. The apparatus of claim 5, wherein the angular distributionaltering element is a diffractive surface having a radially symmetriclens function and a negative axicon function.
 8. The apparatus of claim1, wherein the optical fiber is a multi-mode GRIN fiber and the opticalelement couples light into higher order modes of the multi-mode GRINfiber to reduce differential mode delay.
 9. An apparatus for couplinglight from a light source at an input plane into an optical fibercomprising an optical element which couples light from the light sourceto the optical fiber and which returns light traversing the opticalsystem back towards the input plane such that the returning light doesnot substantially overlap with an output of the light source in theinput plane, wherein the optical element is a diffractive surface havinga radially symmetric lens function and a negative axicon function. 10.An apparatus for coupling light from a light source at an input planeinto an optical fiber comprising an optical element which couples lightfrom the light source to the optical fiber and which returns lighttraversing the optical system back towards the input plane such that thereturning light does not substantially overlap with an output of thelight source in the input plane, wherein the optical element maps apoint from the input plane into more than one point on the opticalfiber.
 11. An apparatus for coupling light from a light source at aninput plane into an optical fiber comprising an optical element whichcouples light from the light source to the optical fiber and whichreturns light traversing the optical system back towards the input planesuch that the returning light does not substantially overlap with anoutput of the light source in the input plane, wherein the opticalelement couples light to the optical fiber by directing light from thelight source away from a center of the optical fiber.
 12. A system forcoupling light to an optical fiber comprising: a light source at aninput plane; and an optical element which couples light from the lightsource to the optical fiber and which returns light traversing theoptical system back towards the input plane such that the returninglight does not substantially overlap with an output of the light sourcein the input plane, wherein said optical element is a mode matchingelement, wherein light output from the optical element is distributed ina desired angular distribution which is substantially maintained alongthe fiber for more than a depth of focus of the optical element.
 13. Thesystem of claim 12, wherein said mode matching element is a diffractiveelement.
 14. The system of claim 12, wherein said mode matching elementis a refractive element.
 15. The system of claim 12, wherein saidoptical element comprises first and second surfaces, said mode matchingelement being provided on said second surface, further from the lightsource.
 16. The system of claim 15, further comprising an angulardistribution altering element on said first surface.
 17. The system ofclaim 16, wherein said angular distribution altering element provides aring pattern on said second surface.
 18. The system of claim 16, whereinthe angular distribution altering element is a diffractive surfacehaving a radially symmetric lens function and a negative axiconfunction.
 19. A system for coupling light to an optical fibercomprising: a light source at an input plane; and an optical elementwhich couples light from the light source to the optical fiber and whichreturns light traversing the optical system back towards the input planesuch that the returning light does not substantially overlap with anoutput of the light source in the input plane, wherein the optical fiberis a multi-mode GRIN fiber and the optical element couples light intohigher order modes of the multi-mode GRIN fiber to reduce differentialmode delay.
 20. A system for coupling light to an optical fibercomprising: a light source at an input plane; and an optical elementwhich couples light from the light source to the optical fiber and whichreturns light traversing the optical system back towards the input planesuch that the returning light does not substantially overlap with anoutput of the light source in the input plane, wherein the opticalelement is a diffractive surface having a radially symmetric lensfunction and a negative axicon function.
 21. A system for coupling lightto an optical fiber comprising: a light source at an input plane; and anoptical element which couples light from the light source to the opticalfiber and which returns light traversing the optical system back towardsthe input plane such that the returning light does not substantiallyoverlap with an output of the light source in the input plane, furthercomprising a power monitor for the light source.
 22. The system of claim21, wherein said power monitor monitors light output from a front of thelight source.
 23. The system of claim 22, further comprising adeflecting element that deflects a portion of the light output from thefront of the light source to the power monitor.
 24. A system forcoupling light to an optical fiber comprising: a light source at aninput plane; and an optical element which couples light from the lightsource to the optical fiber and which returns light traversing theoptical system back towards the input plane such that the returninglight does not substantially overlap with an output of the light sourcein the input plane, wherein the optical element maps a point from theinput plane into more than one point on the optical fiber.
 25. A systemfor coupling light to an optical fiber comprising: a light source at aninput plane; and an optical element which couples light from the lightsource to the optical fiber and which returns light traversing theoptical system back towards the input plane such that the returninglight does not substantially overlap with an output of the light sourcein the input plane, wherein the optical element couples light to theoptical fiber by directing light from the light source away from acenter of the optical fiber.